Latent-image formation in tabular AgBr grains: experimental studies

摘要

Five tabular-grain AgBr emulsions of varying grain thickness were studied. Two were chemically sensitized in the presence of a blue spectral sensitizing dye, whereas the other three were chemically sensitized in the presence of a green spectral sensitizing dye. A companion set of emulsions chemically sensitized in the absence of dye was also prepared. Internal image development of the unsensitized emulsions showed substantial internal image in one emulsion, but minor amounts in the other emulsions. After chemical sensitization, there was no detectable internal image in any of the emulsions. Reciprocity failure data from 10~(-4) to 10~3 s showed that the emulsions sensitized in the presence of dye had little if any high-irradiance reciprocity failure, suggesting the minimum developable size of the latent image was three atoms for the development conditions used. Low-irradiance reciprocity failure commenced at 0.1-1 s. Long wavelength sensitivity studies showed that the chemical sensitization generally enhanced the sensitivity of three spectral regions in the emulsions sensitized in the absence of dye—550, 650 and 750 nm. These spectral regions are suggested to coincide with three distinct states of the sensitizer centres. Data for the emulsions chemically sensitized in the presence of dye were limited owing to the interference by dye absorption. The temperature dependence of the long wavelength sensitivity showed the activation energy for this process increased as the wavelength increased. Quantum sensitivity measurements were also made at the midpoint of the D-logE curve using 0.1 s exposures. Neglecting the polydisperse nature of these emulsions, values were 10-19 absorbed photons/grain for 400-nm exposures and 13-27 absorbed photons/grain for spectral exposures. An energy-level diagram was constructed for the emulsions sensitized in the absence of dye using their measured activation energies and the photon energies of the three spectral regions. The 550 centre is most likely a single-sulphide or single-selenide centre, with an unknown gold content and provides a shallow electron trap (0.1 eV maximum depth). The compositions of the 650 and 750 centres are most likely multiple sulphide or selenide or sulphide-selenide with unknown gold content. They provide deeper electron traps of depth 0.225-0.425 eV (650 centre) and 0.45-0.65 (750 centre), with the 650 centre probably the dominant of the two in terms of concentration.